1. Field of the Invention
The invention relates to a digital-analog converter with self-calibration current sources. These converters are commonly used in integrated circuits made by CMOS technology but may also be used in other types of technologies.
Digital-analog converters can be made in several ways. For reasons of technology and precision, there are converters using parallel-connected current sources whose output is directed either towards an output of the converter or towards a reference terminal.
The current sources are generally formed by a multiple current mirror whose output transistors are either all identical or proportional by powers of two. When it is sought to have very precise converters, i.e. when it is sought to convert more than 10 bits, then it is necessary to calibrate the sources constantly. Furthermore, combined systems are often used in which a first technique is used for the most significant bits and a second technique is used for the least significant bits.
2. Description of the Related Art
For a clearer understanding of the problem solved by the present invention, reference may be made to FIG. 1. FIG. 1 shows n identical sources S.sub.1 to S.sub.n where n=2.sup.N -1. These n sources are connected to commutators C.sub.1 to C.sub.n that connect the output of each source either to the output of the converter to give a current Iout or to a reference voltage, for example the ground. The different connections set up by the commutators C.sub.1 to C.sub.n are controlled by a thermometrical type decoder that validates the number of sources, among the sources S.sub.1 to S.sub.n, that are to be connected to the output of the converter as a function of the number present at the input of the converter, encoded on N bits that have to be converted.
For combined systems, there is added, for example, a source S.sub.n+1 to which there is connected a current divider 1 that is slightly less precise but far more compact than the device described here above. The current divider 1 is generally used to convert the M least significant bits and the n sources S.sub.1 to S.sub.n are used to convert the N most significant bits present at the input of the converter.
If fixed sources are used, the precision of the sources is related to the dimensions of the transistors used and is limited to approximately one per thousand. If it is sought to make conversions of numbers with more than 10 bits, it is necessary to use sources that are calibrated during the working of the converter. The sources used may for example be those described in the article by D. Wouter and J. Groeneveld, "A Self-Calibration Technique for Monolithic High-solution resolution D/A Converter", in IEEE Journal of Solid State Circuits, Vol. 24, No. 6, 6th Dec. 1989. So as not to interrupt the operation of the digital-analog converter, an additional source S.sub.n+2 is added to replace each of the sources S.sub.1 to S.sub.n+1 when one of them gets calibrated. The additional source S.sub.n+2 is connected to the output of each of the sources S.sub.1 to S.sub.n+1 by means of the switches I.sub.1 to I.sub.n+1.
The operation of the device of FIG. 1 is relatively simple. A shift register, associated with the converter or common to several converters present in one and the same chip, selects one source among the sources S.sub.1 to S.sub.n+2. A branch on the calibration command of the sources S.sub.1 to S.sub.n+1 is connected to the switches I.sub.1 to I.sub.n+1 in such a way that when one of the sources S.sub.1 to S.sub.n+1 gets calibrated, it is automatically replaced by the source S.sub.n+2.
As described here above, problems appear. A first problem arises out of the switch-over noise at the output of the converter which is generated by automatic action on at least one of the switches I.sub.1 to I.sub.n, a second problem comes from the cumbersome nature of the wiring of the n switches I.sub.1 to I.sub.n and of the control wires associated with these switches. A third problem is that, whatever the calibrated source, there is at least one commutation on one of the switches I.sub.1 to I.sub.n+1 which tends to produce current variations on the additional source S.sub.n+2.
The invention proposes to overcome the three problems raised here above by using a commutator that will directly connect the additional source of the output.